The invention contemplates UV-absorbing compositions including films and coatings, and multilayer articles prepared therefrom.
Ultraviolet radiation (UV) can cause degradation of certain materials if exposed. Chemical materials known as ultraviolet absorbers, or UVAs, can be used to protect materials from the damaging effects of UV radiation. A UVA can be incorporated into a material to protect that material from UV radiation, or, a composition that contains UVA can be applied to a UV-sensitive substrate to protect the substrate.
Protective coating compositions, sometimes referred to as xe2x80x9ctopcoats,xe2x80x9d can be applied to outdoor-durable materials such as signs based on flexible substrates and optionally having applied graphics, where the coating functions to inhibit dirt buildup or dirt penetration, as a barrier to water, to prevent plasticizers or other ingredients from migrating out of the substrate, or to allow ease of cleaning. A topcoat can include polymeric materials (e.g., a fluoropolymer to provide dirt resistance or cleanability), as well as stabilizers to protect the topcoat or the substrate from degradation, e.g., due to UV radiation. Degradation may involve yellowing, embrittlement, or loss of clarity, gloss, or water resistance.
Unfortunately, while it can be desirable to incorporate a UVA into a protective coating, UVAs can cause some difficult problems. One problem is the relative impermanence of UVAs in many chemical compositions. Non-reactive UVAs can be included in a chemical composition as a dispersed compound, not chemically attached to any other component. These UVAs can be lost from a composition by volatilization during processing (e.g., drying), or by otherwise migrating to the surface of a composition followed by removal as dust or wash off. Loss of the UVA leaves the composition and its substrate less protected from ultraviolet radiation, allowing UV radiation to degrade the composition or substrate. One imperfect remedy to this problem is to include larger amounts of UVA in a composition.
A further problem with UVAs is that they can be incompatible with different polymeric materials (e.g., fluoropolymers). This incompatibility can lead to instability (e.g., thermodynamic instability) or water sensitivity of the composition, which can cause a loss of physical or optical properties, including loss of clarity or increased fogginess. Incompatibility can also cause increased or accelerated loss of UVA by migration, bleeding, or blooming.
Attempts to incorporate UVAs into chemical compositions such as topcoats have been met with a variety of frustrating results, especially when the composition includes an ingredient that is incompatible with the UVA, as are many fluoropolymers. There is a general need to identify ultraviolet absorbing materials and compositions, and also to identify materials that can be used to prepare UV-absorbing compositions such as films and coatings. There is a further need to incorporate UVAs into chemical compositions that contain other materials with which the UVA may not be compatible, wherein the UVA becomes a relatively permanent component of the composition, and wherein the composition is relatively thermodynamically stable, to provide long-term protection from ultraviolet radiation.
The invention provides multilayer articles comprising a flexible substrate and an ultraviolet radiation absorbing multiphase polymeric composition. The multiphase polymeric composition comprises a polymeric core phase and a polymeric shell phase wherein the polymeric core phase includes ultraviolet absorber.
Preferred multiphase polymeric compositions can exhibit thermodynamic stability and UVA retention, thereby exhibiting time-stable protection from ultraviolet radiation, with lasting physical and mechanical properties. This can be true even if the UVA is used in combination with another material that is not compatible with the UVA.
The multiphase polymeric composition can comprise phase domains of a polymeric core phase, a polymeric shell phase, and a polymeric film-forming phase, wherein the morphology is such that the polymeric core phase does not substantially contact the polymeric film-forming phase, but both of these phases contact the polymeric shell phase, which separates the other two phases. It has been found that compositions having this preferred morphology, particularly if the core particle (or a component thereof) is not compatible with the polymeric film-forming material, can exhibit improved initial physical properties, which can be maintained with aging, as compared to chemical compositions containing chemically identical ingredients, in identical amounts, but exhibiting a different morphology. Such properties can include flexibility, lasting protection from ultraviolet radiation due to reduced migration loss (e.g., bleeding or blooming) of the UVA (especially if the UVA is not chemically attached to the core polymer) thermodynamic stability; and improved resistance to water (e.g., reduced water sensitivity).
A particular advantage of the this preferred morphology is that it allows the use of a core particle or a component thereof, e.g., a polymer or UVA, to be used in a composition that contains another material (e.g., a polymeric film-forming material) that is incompatible with the core particle or core particle component. If a core particle or its component is incompatible with a polymeric film-forming material, the shell material can shield the core particle phase from the incompatible phase, thus preventing the consequences otherwise associated with including the core particle with an incompatible material, and thereby achieving a thermodynamically stable composition.
Multiphase polymeric compositions having the described morphology can be prepared by various methods. For instance, such a multiphase composition can be prepared from a latex that includes polymeric particles having a core/shell structure, wherein the core comprises a UVA, and particles of polymeric film-forming material. This can be accomplished by forming the latex into a film or coating and allowing the latex to dry. Alternatively, such a multiphase composition can be prepared by spray-drying the latex form a powder of agglomerate particles, powder coating the agglomerate particles e.g., onto a flexible substrate, and fusing the coated powder.
An aspect of the invention relates to a multilayer article comprising: a flexible substrate, and a multiphase polymeric composition comprising a polymeric core phase, a polymeric shell phase, and a polymeric film-forming phase, wherein the polymeric core phase comprises an ultraviolet absorber, wherein the polymeric core phase and the polymeric film-forming phase do not substantially contact one another, but both the polymeric core phase and the polymeric film-forming phase contact the polymeric shell. The multiphase polymeric composition can block or absorb ultraviolet radiation and can exhibit desirable cleanability properties, and desired mechanical or physical properties such as flexibility.
Another aspect of the invention relates to a method of preparing a multilayer article comprising a flexible substrate and a multiphase polymeric composition, wherein the multiphase polymeric composition comprises a polymeric core phase, a polymeric shell phase, and a polymeric film-forming phase, and wherein the polymeric core phase comprises an ultraviolet absorber. The method includes steps comprising: powder coating onto a flexible substrate polymeric materials comprising the polymeric core phase, the polymeric shell phase, and the polymeric film-forming phase, and fusing the polymeric materials to form a multiphase polymeric composition wherein the polymeric core phase and the polymeric film-forming phase do not substantially contact one another, but both the polymeric core phase and the polymeric film-forming phase contact the polymeric shell phase.
As used herein, the following terms shall be given the recited meanings:
The term xe2x80x9cthermoplasticxe2x80x9d means materials that soften or flow upon exposure to heat and/or pressure. Thermoplastic is contrasted with xe2x80x9cthermoset,xe2x80x9d which describes materials that react irreversibly upon heating so that subsequent applications of heat and pressure do not cause them to soften or flow.
xe2x80x9c(Meth)acrylatexe2x80x9d means either acrylate or methacrylate.
xe2x80x9cPhasexe2x80x9d is used in a manner not inconsistent with its generally accepted meaning in the chemical art, for instance to refer to a discrete, typically homogeneous component of a chemical composition. xe2x80x9cDomain,xe2x80x9d when referring to a phase of a multiphase composition, refers to individual, continuous or discontinuous, microscopic or macroscopic portions of a phase within the multiphase composition. Examples of discontinuous domains include the following, as illustrated in FIG. 3: individual core particle domains; polymeric shell domains; and fluorochemical domains.
In a multiphase polymeric composition, if individual domains of one phase have an affinity for each other (there is an affinity between separate, individual domains of the same phase) that is significantly greater than the affinity between domains of that phase and domains of a chemically different phase of the composition which the first phase contacts, individual domains of the like phases will be thermodynamically driven to combine with each other, causing polymer segregation and growth in domain size of like phase domains. Phases of a multiphase polymeric composition are referred to as xe2x80x9cincompatiblexe2x80x9d if, while both are contained in a multiphase composition, they tend to combine with each other within the composition to form larger phase domains. If domains of a like phase of a multiphase polymeric composition do in fact form larger-sized phase domains, i.e., the composition suffers gross symptoms of polymer segregation, the multiphase polymeric composition is considered to be xe2x80x9cthermodynamically unstable.xe2x80x9d
Gross symptoms of polymer segregation can include decreased optical clarity, loss of flexibility, embrittlement, and other effects. Polymer segregation within a multiphase polymeric composition can be considered to occur upon a significant loss of clarity or flexibility, or a significant increase in brittleness. These properties can be measured by known methods.
Brittleness can be measured by measuring elongation, with a 20% decrease in elongation being considered significant and indicating polymer segregation and thermodynamic instability.
Clarity can be measured using a Hazemeter. If the result of a haze measurement according to ASTM D1003, upon aging, increases by 10 percentage points, e.g., 5 percentage points or more (e.g., from a reading of 0% to 5%, see the Examples Section and Table 1), it is considered to have lost a significant amount of clarity, and is considered to be thermodynamically unstable.
Flexibility of a multiphase polymeric film composition applied to a flexible substrate can be detected by the following test: samples of the film (approximately 20 to 50 micrometers in thickness) coated onto a substrate (approximately 500 to 600 micrometers in thickness, e.g., 3M PANAFLEX(trademark) 930 Sign and Awning Substrate) (optionally aged e.g., for 1 week at 150 F.). Two samples of the coated film can be cooled to room temperature (about 25xc2x0 C.), and then bent and creased, one in a direction to cause compression of the coating, and another to cause extension. Creasing can be a fold to 180 degrees, followed by heavy finger pressure along the fold. If the coating visibly cracks, the coating is considered to have lost flexibility. To more easily see cracks, a permanent marker can be applied to the creased coating, and the marker can be washed off with isopropyl alcohol. If a crack is present, the marker will stain the crack and remain visible after washing; if no cracks are present, the marker will wash clean.